1. Field of the Invention
This invention relates to the field of filter elements constructed with poly(tetra-fluoro ethylene); i.e., "PTFE" resin membranes. Membranes made of poly(tetra-fluoro ethylene), such as Teflon.RTM., are used for filtration in the fields of foods, semi-conductors, and the like. These filters are also used in fields where highly reactive chemicals are in use.
2. Related Case
3. The Prior Art
Poly(tetra-fluoro ethylene) membrane filters, in a variety of constructions have proven attractive in a variety of fields where the unique properties of poly(tetra-fluoro ethylene) resins afford particular advantage. Among the virtues of poly(tetra-fluoro ethylene) materials are the chemical inertness and high temperature characteristics.
Poly(tetra-fluoro ethylene) membranes are based on stretching, often biaxially, a film of the polymer. Such materials are commercially available from a variety of sources, in a range of properties such as pore diameter, thickness, engineering properties and the like.
Filter cartridges of pleated poly(tetra-fluoro ethylene) membranes are known in the art and have proven to be of considerable use, both for the particular properties of the poly(tetra-fluoro ethylene) membrane and for the facility and convenience of the cartridge form.
As those of ordinary skill in the art understand, it is desirable to minimize the use of materials other than poly(tetra-fluoro ethylene) in the fabrication of such pleated membrane cartridges. While it has not proven possible to date to eliminate all materials other than poly(tetra-fluoro ethylene) in such cartridges, it has been possible to substitute other fluorocarbon polymers which offer some of the properties of poly tetra-fluoro ethylene), in combination with true thermoplastic characteristics which enable the fabrication of pleated membrane cartridges.
Poly(tetra-fluoro ethylene) membranes require structural support to withstand the stresses of filtration. In pleated cartridges, the membrane is ordinarily sandwiched between porous webs of other materials which provide adequate structural characteristics to protect the membrane.
Such pleated membrane filter cartridges made wholly of fluorocarbon resins are known in the prior art as illustrated in U.S. Pat. No. 4,663,041, issued to Miyagi, et al. Miyagi, et al. teach the use of a poly(tetra-fluoro ethylene) membrane as the essential filter medium. The filter element is produced by pleating a sandwich-form shape comprising a filter membrane made of poly(tetra-fluoro ethylene) resin and fluorocarbon thermoplastic polymer net mesh supports extruded of thermoplastic fluorocarbon resin superimposed on both surfaces thereof. The material is then formed into a pleat and hot melt bonded at the edges of the pleats into a cylinder. Such extruded mesh netting of poly(tetra-fluoro ethylene) is not known, and other fluoropolymers, such as TFE, PFA, FEP, ETFE, PVDF, ECTFE, PVT, and blends of such thermoplastic fluoropolymers are employed.
Another example of a prior art fluorocarbon filter is found in United Kingdom patent application GB 2,152,399A, Flaherty, et al., published Aug. 7, 1985, wherein a filter medium is constructed using a membrane and screen sandwiched together. This disclosure is quite close to Miyagi, et al., supra, with whom Flaherty, et al., is presently involved in an Interference. The extruded fluoropolymer screen provides structural support for the poly(tetra-fluoro ethylene) membrane. The import of Flaherty, et al., is cumulative to that of Miyagi, et al.
The extruded fluorocarbon thermoplastic polymer mesh or net screens employed by Miyagi, et al., and by Flaherty, et al., have several disadvantages. The thermoplastic fluoropolymer is not poly(tetra-fluoro ethylene), first and foremost, and cannot offer the chemical inertness, thermal stability and related properties afforded by poly(tetra-fluoro ethylene). In addition, such extruded screens are relatively thick, on the order of about 0.3 mm to about 0.75 mm. As a consequence, the effective surface area of the poly(tetra-fluoro ethylene) membrane in the pleated structure is limited by the bulk of the non-poly(tetra-fluoro ethylene) support.
Another attempt to construct a porous fibrous fluorocarbon filter is shown in U.S. Pat. No. 4,716,074, issued to Hurley, et al. In this structure, poly(tetra-fluro ethylene), PTFE fibers are assembled into a web with a fluorocarbon binder. The fluorocarbon binder is cured in order to hold the poly(tetra-fluoro ethylene) in place.
The poly(tetra-fluoro ethylene) web taught by Hurley, et al., employs lesser amounts of material other than poly(tetra-fluoro ethylene), and is relatively thin. The fluoropolymer adhesive relied upon to bond the web has the great disadvantage of becoming dislodged, however, as an inherent characteristic of adhesives or other materials bonded to the surface of poly(tetra-fluoro ethylene) materials, noted for their "non-stick" properties. As a consequence, the Hurley, et al., web results in a contamination of the filtrate with loosened bits of the fluoropolymer adhesive and, in some cases, with loosened poly(tetra-fluoro ethylene) fibers. In the uses for which such cartridges are most often employed, such filtrate contamination is unacceptable.
In Japanese patent JP 63165598, there is disclosed a method of construction of a fluorofiber paper. A fluorofiber paper (PTFE paper) is constructed by orienting fluorofibers in random directions and bonding them together by hot melt bonding. The manufacturing method comprises making a fluorofiber containing paper by subjecting the fluorofibers and temporary extractable binding agent to wet-mix paper making, drying, and then hot-pressing the paper at the softening point of the thermoplastic fluorofiber or at a higher temperature to hot-glue them together. Next, the temporary extractable binding agent is removed by dissolving it with a solvent and washing, and re-drying if necessary. By this method, a fluorofiber web is constructed. The fluorofiber disclosed can be PTFE, TFE, PFA, FEP, ETFE, PVDF, ECTFE, PVT, and blends thereof.
This application discloses that a fluorofiber paper is obtained which exhibits superior characteristics of fluororesins such as heat resistance, chemical resistance, noncombustibility, electrical insulation, and non-stickiness. The paper may be used in various applications such as filter media, heat insulation materials, spacers, insulation materials, and non-sticking materials in diverse industrial fields. The application, however, makes no disclosure as to the strength of poly(tetra-fluoro ethylene) paper or of properties suitable for support of membranes in pleated cartridge construction. Because poly(tetra-fluoro ethylene) is not readily processed by thermoplastic techniques, it is unclear that poly(tetra-fluoro ethylene) fibers are contemplated for use per se or only in blends with other thermoplastic fluorocarbon polymers.
As no example is given which employs poly(tetra-fluoro ethylene) fibers, and little information or guidance is provided concerning the use of poly(tetra-fluoro ethylene) fibers in the invention, it is not apparent from Japanese Patent JP 63165598 that the requirements of a membrane support medium are provided. The suggestion of usage as a filter medium implicitly suggests, to those of ordinary skill in the art, that a support should be considered if the paper is employed in such usage.
The art has recognized the limitations and shortcomings of the fluoropolymer mesh support employed by Miyagi, et al., and by Flaherty, et al., but efforts eliminate non-PTFE components and constituents completely from the support have not heretofore succeeded.
In the Hurley, et al., effort to improve on the extruded mesh support, the physical demands imposed on the support structure in filtration service led to reliance on adhesives to bond poly(tetra-fluoro ethylene) fibers into a web. As already noted, the familiar non-stick properties of poly(tetra-fluoro ethylene) materials has led to sloughing of particles of the adhesive material, and ultimately of poly(tetra-fluoro ethylene) fibers, into the filtrate. Such filtrate contamination is quite unacceptable in most applications for poly(tetra-fluoro ethylene) membrane filter cartridges and has severely circumscribed the acceptance of the Hurley, et al., product.
The desirability of an all poly(tetra-fluoro ethylene) filter medium, based on both membrane and support made from all-PTFE materials, and free of other, non-PTFE, materials has, until the present invention, remained unfilled.